The "common cold" is just that -- common. Adults get an average of three colds per year, and children can get as many as eight to 10.
Over-the-counter antihistamines, decongestants and pain relievers can help alleviate the sneezing, congestion, runny nose and sore throat -- but medical science has yet to find a cure.
A walk down the cold and flu aisle at the drugstore, however, would seem to tell another story. All manner of supplements and homeopathic treatments that claim to be able to stop a cold dead in its tracks are available.
Among the most popular: lozenges, nasal sprays and nasal gels containing the essential element zinc. Regardless of their mode of delivery, all of these zinc treatments claim to shorten the duration and lessen the severity of a cold if taken at the first sign of symptoms.
But the effectiveness of zinc treatments on the cold is unclear.
On the one hand, the websites and product literature for zinc cold remedies cite numerous clinical studies that have shown that zinc can shorten the duration of the common cold, reduce the severity of symptoms, or both.
On the other, a deeper dig into the medical literature reveals that for every study appearing to show zinc is effective in treating the common cold, another shows it has no effect at all.
To answer the question of zinc's effectiveness once and for all, Tom Caruso, a medical student in his last year of training at the Stanford University School of Medicine, decided to bring together and analyze all of the available research on zinc as a treatment for the common cold. "I did this review to search for the truth," he said. So many friends and family members were trying zinc treatments for their colds.
The study, conducted with Stanford professor of medicine Dr. Charles Prober and Dr. Jack Gwaltney, professor of medicine at the University of Virginia School of Medicine, gathered every study on zinc and the common cold published between 1966 and 2006 -- 105 studies all told.
Caruso first excluded studies that were conducted by artificially inducing a cold (which wouldn't mimic the way colds are caught in real life) and studies that examined preventing colds. He analyzed the rest to assess whether they were conducted in a way that would give valid results. ("A lot of studies are not designed well," he said.)
Only 14 of the 105 were so-called randomized, placebo-controlled clinical trials, considered the gold standard of medical research.
The study, published in the September issue of the journal Clinical Infectious Diseases, found that exactly half of those 14 studies reported a positive effect of zinc on the common cold, with the other half reporting no effect.
Then the authors applied a further set of quality-control criteria -- ones they had decided would be necessary for quality results before they even began their analysis. (These included 11 different considerations, such as whether the scientists had adequately ensured that participants were randomly assigned to receive zinc or a placebo and that the study was designed to give statistically significant results.)
After this was done, only four of the 14 studies made the cut: two that examined zinc lozenges, one that studied a nasal spray and one that studied a nasal gel.
Three of the four found that zinc had no effect on the common cold. Only the study looking at the effectiveness of nasal gel found an effect: The patients who received the zinc nasal gel had colds that lasted an average of two-and-a-half days, whereas the patients who received a placebo were sick for an average of nine days.
Caruso said that this one positive study is not enough to prove that a zinc nasal gel works, "but maybe there is something there," he said.
He added that further, well-designed studies should be conducted to try to replicate the findings.
Because of the general lack of evidence for zinc's effectiveness, Caruso said he wouldn't recommend trying it as a cold treatment.
"But if you really have to do it, go with the gel," he said.
Even if zinc does work against the common cold, no one really knows how.
The cold remedies containing zinc are not marketed to correct a zinc deficiency in the body, although a lack of zinc can impair the immune system. Instead, zinc cold treatments claim to work by delivering zinc to the scene of the crime -- nasal tissue that has been infected by cold viruses.
Scientists propose that zinc fights colds by keeping rhinoviruses, the main type of cold virus, from attaching to cells in the nose, the idea being that if they can't attach, they can't infect. Thus, the idea goes, if someone began zinc treatment during the early stages of a cold, he or she might reduce the number of viruses that infected the nasal cells and therefore shorten the cold's duration.
But University of Virginia pediatrics professor Ronald Turner, who has studied the effect of zinc on the common cold, says that this explanation of why zinc treatments might work against the cold virus is "entirely theoretical" and "has never been demonstrated in an experiment."
The most common side effects of zinc cold treatments are an unpleasant taste in the mouth and an upset stomach. Some people who have used the nasal sprays and gels report a stinging sensation.
In January 2006, Matrixx Initiatives, the makers of the zinc nasal gel Zicam, settled a class action lawsuit with people who contended that the product had caused them to lose their sense of smell, some after just one use of the product. (The product instructions say to use the gel every four hours from the first sign of a cold until 48 hours after symptoms subside.)
A news release from Matrixx says that settling the suit was a business decision and that the company does not accept that its product damages the sense of smell.
Turner said that the evidence that zinc products could cause a loss of the sense of smell is only anecdotal and that there have not been any systematic scientific studies proving that this can happen.
But, he added, given that there's no clear evidence that zinc can help fight colds, even anecdotal reports of it impairing the sense of smell are "a little worrisome."